Today, the data usage of wireless communication networks is increasing by geometric progression. In accordance with this, the beyond 4-Generation (4G) mobile communication technology has to support Gigabyte-class communication even in the outdoor environment. The most highlighted candidate technology for this is a beamforming technology of a millimeter (mm) band. Compared to an existing low frequency band, the millimeter band has the merit of being capable of using a broadband. But, the millimeter band has the demerit of high channel attenuation. The beamforming is a technology capable of increasing a strength of a signal in a specific direction by using a plurality of antennas. In the past, many researches were made on the beamforming. So, the beamforming can integrate the plurality of antennas in a small region as a length of a wavelength decreases at a high frequency band. Accordingly, the beamforming technology is again attracting attention as a significant technology capable of overcoming the channel attenuation of the millimeter band.
The beamforming can be implemented anywhere in a digital end, i.e., a baseband, after Digital to Analog (D/A) conversion, or a Radio Frequency (RF) end. When the beamforming is implemented in the digital end, it is easy to adjust a beamforming coefficient, that is, a strength and phase of a signal. But, in this case, there are difficulties in cost and implementation because an RF chain joining baseband—D/A—RF should be provided every antenna. In contrast, when the beamforming is implemented in the RF end, it incurs a low price but has a difficulty in minutely adjusting the strength and phase of the signal.
Hybrid beamforming is a method of transmitting multiple data through a digital precoder and getting a beamforming gain through an RF beamformer, by constructing a baseband by only a few RF chains and again coupling the RF beamformer every each RF chain. Accordingly, the hybrid beamforming can achieve suitable trade-off compared to the aforementioned digital beamforming and analog beamforming technology.
The hybrid beamforming can be coupled with an Orthogonal Frequency Division Multiplexing (OFDM) system, and can be variously implemented in structure. For example, RF beamformers of a plurality of RF chains can be connected to a single physical array antenna through an adder. In this case, the array antenna connected with the RF beamformer of each RF chain is identical. Accordingly, a correlation of an effective channel of a baseband basis can greatly increase in accordance to a beam coefficient of each RF beamformer. In other words, a correlation between beams used for beamforming is varied in accordance to the beam coefficient. The variation of the correlation between the beams can make performance worse or better in accordance with a used Multiple Input Multiple Output (MIMO) mode. Therefore, the hybrid beamforming structure has to propose an alternative for suitably selecting a beam that will be actually used for data transmission/reception in consideration of the correlation between the beams.